1. Field of the Invention
The present invention relates to lens control adapted for use in a lens of inner focus type.
2. Related Background Art
FIG. 1 illustrates a generally employed simple structure of an inner focus type lens system. There are provided a fixed first lens group 101; a second lens group 102 for zooming (hereinafter called zooming lens); a diaphragm 115; a fixed third lens group 103; a fourth lens group 104 having focusing function and so-called compensating function for compensating the displacement of the focal plane resulting from zooming (hereinafter called focus/compensating lens); and an image pickup plane 119 of an image pickup device.
In the lens system as shown in FIG. 1, as already known, the position of the focus/compensating lens 104 for focusing on the image pickup plane 119 varies depending on the distance to the object, even for a given focal length, because the lens 104 serves for both functions of focusing and compensation.
FIG. 2 shows the continuous plotting of the position of the focus/compensation lens 104 for focusing to the image pickup plane, when the object distance is varied at each focal length. In FIG. 2, the abscissa indicates the position of the zooming lens or the focal length, while the ordinate indicates the position of the focus/compensating lens, and the curves are focusing trajectories corresponding respectively to different object distances. A zooming operation without image blur can be achieved by selecting one of the plural trajectories shown in FIG. 2, corresponding to the object distance, and by moving the focus/compensating lens 104 according to thus selected trajectory.
A lens system of the front lens focusing type contains a compensating lens independent from the zooming lens, and both lenses are mutually linked by a mechanical cam. In case of manually varying the focal length by a manual zooming knob attached to a ring of said cam, the cam ring is rotated, following any movement of said knob, and the zooming lens and the compensating lens move along the cam grooves of said cam ring. Consequently, as long as the focusing lens is properly positioned, the above-mentioned operation does not generate image blur.
On the other hand, in the lens system of the inner focus type explained above, the control is generally executed by storing the plural trajectories shown in FIG. 2 in a certain form in a lens controlling microcomputer, then selecting one of said plural trajectories according to the positions of the focusing lens and the zooming lens, and effecting the zooming operation, tracking thus selected trajectory.
In such lens control, the positions of the lenses have to be read with a certain high level of precision, because the position of the focusing lens with respect to the position of the zooming lens is read from a memory device and is utilized for the lens control. As will be apparent from FIG. 2, when the zooming lens moves with a constant or nearly constant speed, the inclination of the trajectory of the focusing lens varies depending on the focal length. This means that the speed and direction of movement of the focusing lens varies continuously. Thus, if a stepping motor is employed in the actuator for the focusing lens, there is required a highly precise response over a range from one to several hundred cycles per second.
In the lens system of such inner focus type, a stepping motor is being commonly employed as the actuator for the focusing lens group, in order to meet the above-mentioned requirement. The stepping motor rotates in complete synchronization with the stepping pulses released, for example, from a lens controlling microcomputer with a constant stepping angle per pulse, and is capable of providing a high speed response, a high stopping precision and a high positional precision.
Also since the rotation angle of the stepping motor can be given by the number of stepping pulses, the pulses can be directly used as an incremental position encoder, and no additional position encoder is therefore required.
In such configuration, however, since the position is to be detected by the stepping motor itself, the stepping pulse counter has to be reset to a specified count prior to the position detection. If the count of said counter is incorrect, a proper trajectory cannot be read from the memory corresponding to the combined coordinates of the positions of the zooming lens and the focusing lens, and an image blur results since a proper trajectory tracking cannot be attained during the zooming operation.
Said resetting operation is usually executed by moving the focusing lens to a predetermined position immediately after the start of power supply but before the start of normal operation, and then setting the counter to a predetermined value. It is to be noted, in such operation, that the focusing lens starts to move from a position at which the power supply is started. Consequently, if the photographer turns off the power supply in order to avoid the consumption of the battery after adjusting the focus and the image angle and turns on the power supply again afterwards, there may result a situation that the lens is no longer in the previously adjusted position, or, even if the focusing lens is returned to the previous position, the position does not necessarily correspond to the distance of the object which the photographer wishes to take. Also if said resetting position is significantly different from the position of the focusing lens corresponding to the frequently used object distance, a significantly blurred image will be displayed for a while after the start of normal operation, unless a certain measure is taken for the returning of the focusing lens after the resetting operation. Furthermore, if the display is muted until the image blur is compensated to a certain level for example by the automatic focusing function, there will be required a long time until the normal operation is enabled, after the start of the power supply.